Generally, a line-head-type inkjet image-forming apparatus has an inkjet head having a plurality of nozzles arranged in one or more rows over the entire width of the image to be formed. The inkjet head is fixed, under which a recording medium such as a paper or fabric sheet is fed in a direction crossing the nozzle arrangement direction, while ink is ejected from the inkjet head to form the desired image. Such a line-head-type inkjet image-forming method is advantageous in terms of mass production of high-quality images.
On the other hand, such an image-forming method, in which the inkjet head is fixed relative to the recording medium being fed, can cause image defects such as stripes and uneven density when a failure occurs in any of the nozzles (such as deflected ejection of ink or ejection failure). If the ink used is a gel ink, the image defects can more easily occur because the gel ink has high pinning ability and thus less easily levels out than non-gel inks.
For example, if any one of the nozzles fails to eject the ink, a stripe-like blank (stripe defect) can appear in the resulting image. In the line-head-type inkjet image-forming method, a known technique to prevent the stripe defect includes increasing the amount of ejection of ink from a nozzle adjacent to the nozzle failing to eject ink or further increasing the dot diameter (e.g., refer to Patent Literatures 1 and 2).